Multi-layer piezoelectric actuators constituted from piezoelectric layers and internal electrodes stacked alternately one on another have been known. The multi-layer piezoelectric actuators can be divided into two categories: fired-at-once type and stacked type which has such a constitution as piezoelectric porcelain made of a piezoelectric material and internal electrodes having the form of sheet are stacked one on another alternately. When the requirements to reduce the operating voltage and the manufacturing cost are taken into consideration, the multi-layer piezoelectric actuator of fired-at-once type is more advantageous for the reason of smaller layer thickness and higher durability.
FIG. 24A and FIG. 24B show a multi-layer piezoelectric element of the prior art disclosed in Patent Document 1, which is constituted from a stack 103 and external electrodes 105 formed on a pair of opposing side faces thereof. The stack 103 is formed by stacking piezoelectric layers 101 and internal electrodes 102 alternately one on another. The internal electrodes 102 are not formed over the entire principal surfaces of the piezoelectric layers 101, but have a so-called partial electrode structure. In the stack of the partial electrode structure, the internal electrodes 102 are stacked in a staggered manner so as to be exposed alternately at the left in one layer and then at the right in the next layer. Thus the multi-layer piezoelectric element of fired-at-once type of the prior art has the internal electrodes of partial electrode structure, so that the internal electrode and the external electrode of different polarities do not contact with each other. There is also such a type that every other internal electrode is covered by an insulator on one side of the stack so as to be insulated from the external electrode. In this way, with any of the types described above, the external electrodes 105 are formed so as to be connected to the internal electrode that is exposed on either of a pair of opposing side faces of the stack 103 in every other layer. Inactive layers 104 are stacked on both end faces of the stack 103 in the direction of stacking.
The multi-layer piezoelectric element of the prior art is manufactured as follows. A paste for the internal electrode is printed on a plurality of ceramic green sheets that include the substance used for making the piezoelectric material, and are stacked and fired so as to form the stack 103. Then the external electrodes 105 are formed on a pair of side faces of the stack 103 thereby to make the multi-layer piezoelectric element (refer to, for example, Patent Document 1). The paste for the internal electrode is printed on ceramic green sheets in a predetermined pattern as shown in FIG. 24B.
When manufacturing the multi-layer piezoelectric element of fired-at-once type, it is required to fire the internal electrode 102 and the piezoelectric material 101 to sinter at the same temperature, in order to bond the internal electrode 102 and the piezoelectric material 101 firmly. Optimum compositions of the internal electrode 102 and the piezoelectric material 101 are being studied for satisfying this requirement.
The internal electrode 102 is usually formed from an alloy of silver and palladium and, in order to fire the piezoelectric layers 101 and the internal electrodes 102 at the same time, composition of metals included in the internal electrode 102 has been set typically to 70% by weight of silver and 30% by weight of palladium (refer to, for example, Patent Document 2).
The internal electrode 102 made of a metal compound that includes silver-palladium alloy is capable of suppressing the so-called silver migration in which silver atoms migrate along the device surface. In the case of composition constituted from pure silver, when a voltage is applied between a pair of opposing internal electrodes 102, the silver migration occurs in which silver atoms migrate from the positive electrode to the negative electrode along the device surface. Silver migration occurs conspicuously particularly in an atmosphere of high temperature and high humidity.
In the field of multi-layer ceramic capacitor, it has been proposed in Patent Document 3 to provide voids in an end portion of the internal electrode. This is for the purpose of mitigating the stress generated in the interface between the dielectric layer and the internal electrode layer, thereby to provide a multi-layer ceramic capacitor that is excellent in reliability including the resistance against bending of the substrate and has stability in electrical properties and high performance.
However, unlike the conventional multi-layer electronics components (multi-layer ceramic capacitor, etc.), the multi-layer piezoelectric element experiences a significant deformation (displacement) of the piezoelectric porcelain under an electric field when electric power is supplied. The number of operation cycles is the same as the number of times the piezoelectric porcelain deforms.
Taking this point into consideration, a multi-layer piezoelectric element that frequently undergoes displacement has been constituted to have internal electrode formed in dense structure in order to ensure durability of the internal electrode.
The multi-layer piezoelectric element having such a constitution as described above can be used as a piezoelectric actuator by securing lead wires onto the external electrodes 105 by soldering, and applying a predetermined voltage across the external electrodes 105.    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 61-133715    Patent Document 2: Japanese Unexamined Utility Model Publication (Kokai) No. 1-130568    Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. 2002-231558